1. Field of the Invention
This invention relates to a color filter for a liquid crystal display device and to a photomask to be employed for the manufacture of the color filter. In particular, this invention relates to a color filter for a liquid crystal display device wherein two kinds of photo-spacers differing in thickness are employed.
2. Description of the Related Art
As shown in FIG. 7 for example, the color filter to be employed in a liquid crystal display device is constituted by a black matrix 71, a color pixel 72 and a transparent conductive film 73, all of which being successively deposited on a glass substrate 70. The color filter constructed in this manner can be manufactured as follows. First of all, the black matrix is formed on the glass substrate, then the color pixel is formed in alignment with a pattern of the black matrix, and the transparent conductive film is formed on the color pixel.
Due to the development and practical use of various kinds of liquid crystal display devices, there is an increasing trend to provide the color filter with various supplementary components having various kinds of functions depending on the application and specification of color filter, such supplementary components including, for example, 1) a protective layer (overcoat layer); 2) a transparent portion in the color filter to be employed in a translucent liquid crystal display device; 3) an optical path difference-adjusting layer for making uniform the phase of light passing through a permeative display region and a reflective display region; 4) a light-scattering layer for the reflective display region of color filter; 5) a photo-spacer (protruded portion) having spacer function; 6) an alignment-controlling protrusion for controlling the alignment of liquid crystal molecules; etc.
Especially, in the case of conventional liquid crystal display device, so-called spacer which is formed of transparent spherical particles (beads) or short fiber each made of glass or synthetic resin has been used for forming a gap between substrates.
However, since this spacer is formed of transparent particles, light is permitted to leak out through the spacer on the occasion of displaying black color if the spacer is permitted to enter, together with the liquid crystal, into the pixel. Further, due to the presence of the spacer between the substrates with a liquid crystal material being sealed therebetween, the alignment of liquid crystal molecules in the vicinity of the spacer is disturbed to generate the leakage of light at this portion, resulting in the decrease of contrast, thus raising various problems including adverse influences to the quality of display.
With a view to overcome these problems, there has been developed a technique wherein protrusions exhibiting the spacer function (hereinafter referred to as photo-spacers) are formed on the region of black matrix existing between pixels by means of photolithography method and by making use of a photosensitive resin.
FIG. 8 is a partial sectional view of such a color filter for liquid crystal display device. As shown in FIG. 8, a black matrix 81, a color pixel 82 and a transparent conductive film 83 are successively formed on the surface of glass substrate 80, and a photo-spacer 84 having the spacer function is formed as a protrusion on the regions of the transparent conductive film 83 below which the black matrix 81 is located, thus constituting the color filter 87 for liquid crystal display device. In the case of the liquid crystal display device where the color filter 87 constructed in this manner is employed, since the photo-spacer 84 is formed at the regions of transparent conductive film 83 where the color pixel is substantially not located, it is possible to improve the liquid crystal display device with respect to the improvement of contrast of liquid crystal display.
In the assembling step for creating a liquid crystal display panel through the lamination of the substrate with the color filter for liquid crystal display device, a seal portion (not shown) is formed at first on a peripheral portion of the substrate and of the color filter and then the assembly comprising the substrate and the color filter is placed between an upper platen and a lower platen. Thereafter, a load is applied to the assembly sandwiched between these upper and lower platens to thereby contact-bonding these seal portion and photo-spacer 84. On this occasion, due to the load applied to the assembly, the photo-spacer is elastically deformed more or less and, under the condition where the photo-spacer is deformed in this manner, the setting of gap between substrates is performed.
Even though the gap between the substrates is set in this manner by making use of the photo-spacer, it is desirable to minimize the deformation of the photo-spacer as an ordinary degree of load is applied to the panel and to prevent the plastic deformation and fracture of photo-spacer that may be caused to occur when an excessive load is applied to the panel.
As for the countermeasure to cope with the problems of the plastic deformation and fracture of photo-spacer due to an excessive load, there has been proposed a color filter for a liquid crystal display device wherein two kinds of photo-spacers are provided therein. FIG. 9 is a cross-sectional view schematically illustrating one example of such a color filter for a liquid crystal display device that is provided with two kinds of photo-spacers differing in height. As shown in FIG. 9, this color filter for a liquid crystal display device differs from the color filter constructed as shown in FIG. 8 in the respect that the photo-spacer is constituted by two kinds of photo-spacers, i.e., a main photo-spacer 84a which is relatively large in height and a subphoto-spacer 84b which is relatively small in height.
Among these two kinds of photo-spacers, the main photo-spacer 84a is utilized for setting the gap between the substrates. This main photo-spacer 84a is designed such that it is enabled to deform as a load is applied to the panel and that the configuration thereof is enabled to restore as the load is removed. Further, this main photo-spacer 84a is made elastic such that it can be deformed in conformity with the thermal expansion or thermal shrinkage of the liquid crystal that may be caused to occur due to changes of temperature.
The subphoto-spacer 84b is a photo-spacer which is lower in height than the main photo-spacer 84a. This subphoto-spacer 84b is designed such that when an excessive load is applied to the panel, this subphoto-spacer 84b acts to disperse this excessive load, thereby preventing the plastic deformation and fracture of the main photo-spacer 84a. 
As for the method of forming the main photo-spacer and the subphoto-spacer which differ from one another, it is possible to employ a method to form a photo-spacer having a lower height, wherein a photomask having the same configuration as that of the main photo-spacer but having a smaller aperture than that of the main photo-spacer to thereby control the quantity of light transmittance (see for example, JP-A 2002-189281.
However, according to this method, since the subphoto-spacer is formed by making use of a photomask having a smaller aperture for reducing the quantity of light to be transmitted, the size of subphoto-spacer 84b is inevitably caused to become smaller not only in height but also in diameter as compared with those of the main photo-spacer 84a. If the diameter of subphoto-spacer 84b is small, the press resistance thereof would be inevitably lowered. Therefore, if it is desired to enhance the press resistance of subphoto-spacer 84b, the density of subphoto-spacer (the number of photo-spacer per unit area) is required to be increased. There is however limitations in increasing the density of photo-spacer. Further, since the subphoto-spacer which is small in diameter is vulnerable to peeling in the manufacturing process thereof, it is very difficult to create it unless the production conditions are strictly controlled.
There is also known a method of forming two kinds of photo-spacers differing in height from one another, wherein the subphoto-spacer is created through the control of transmittance of exposure light by applying half tone working as well as gray tone working to a photomask. This method however requires high precision in the working of the photomask, which is rather complicated.